Recently, vehicles of high voltage (approximately 300V) such as hybrid vehicles and fuel-cell powered vehicles have been developed from the standpoint of environmental issues. Such vehicles include a control circuit constituted of a main battery of high direct current voltage and a high voltage circuit. In the case of an accident or the like, the battery must be disconnected from the control circuit since it corresponds to a high direct current voltage. To this end, a direct current relay formed of a mechanical contact is provided between the battery and the control circuit.
In such relays, the cut off speed is extremely low since the arc generated when the high direct current voltage is to be cut off is very large. It was extremely difficult to achieve cut off in a short time. In view of the foregoing, there is known a conventional structure of placing a magnet at the arc generating region to extend the arc by Lorentz force (for example, refer to Japanese Patent No. 3321963).
The direct current relay disclosed in Japanese Patent No. 3321963 includes two pairs of contacts, each contact pair being sandwiched by a pair of magnets arranged so as to be orthogonal to a line connecting the contact pairs. In this relay, the magnets forming a pair are arranged so that the opposite magnetic pole facing each other differ. These pairs of contacts have the contacts provided so that current flows in series when connected.
In accordance with Japanese Patent No. 3321963, the arc generated between the contacts, when each contact pair attains a non-contact state, is distorted to extend on the line connecting the two contact pairs and towards the side opposite to the adjacent contact pair (outer side).
The conventional relay disclosed in Japanese Patent No. 3321963 requires space to ensure sufficient arc extension for immediate relay cutoff since a pair of magnets are disposed corresponding to each contact pair, and the arc is extended outward of these contact pairs on a line connecting the two contact pairs through the action of the magnetic field.
The number of magnets is increased in order to dispose a pair of magnets for each contact pair having the attraction corresponding to the degree of arc extension. This poses the problem that the entire relay is increased in size.
Furthermore, the cost of the relay will become higher since the increased number of pairs of magnets, one pair disposed for each contact pair, will induce further time and effort in the assembly procedure.
Hybrid vehicles and the like employ a system to convert kinetic energy into electric energy to charge the battery at the time of deceleration. Therefore, a backward current (regenerative current) may be generated in the relay. The need arises for a relay to be cut off even in the case where a backward current flows excessively.
However, if the relay is cut off when a backward current is generated in accordance with the configuration of the relay disclosed in Japanese Patent No. 3321963, the arc occurring between the contacts will be distorted towards a region between the two contact pairs by the Lorentz force of the magnet. In this case, each arc will be extended towards an adjacent pair of contacts to be linked together, giving rise to the problem that immediate cutoff cannot be achieved.
Furthermore, superior welding resistance and temperature characteristic are required since the generated heat is great due to the high contact resistance of the contact unit.